CYLINDER PRESSURE... How much do we have?

[ClimberDHexMods]

What is the quantifiable relationship between timing and cylinder pressure?

 

Is there a known point of ignition timing at which further ignition advance will cause cylinder pressure to rise significantly?

 

Is there a certain level of cylinder pressure that is "dangerous"? If so, at what point does cylinder pressure start to become dangerous?

 

What is the relationship between MBT (maximum beneficial timing) and cylinder pressure?

 

Expert knowledge would be hugely appreciated!

[subawang]

The simple answer is is that there is no simple answer. In general, as you advance timing, you will also increase peak cylinder pressures until you hit MBT.

 

Most if not all late model turbo'd cars are not able to run MBT (typically an acronym for maximum brake torque timing), because they are knock limited. So, in general you have no worries in terms of reaching peak cylinder pressures which might push the structural limits. This is unless you are running E85, in which case you might actually be able to run MBT. If that is the case, you most likely want to start looking into building up the internals. But again, there is no simple answer.

[ClimberDHexMods]

Since we know degrees at ignition, and can somewhat figure burn speed by EGT, is it realistic to try to calculate cylinder pressure at various points in the stroke? That's probably above my pay grade, but way better than guessing and fingers crossed...

[NSFW]

It's my understanding that - for a given RPM, load, charge temp, etc - cylinder pressure increases slowly with ignition advance, until you hit the threshold for detonation. At that point, cylinder pressure goes up enormously due to detonation.

 

But it sounds like you want to use cylinder pressure to predict detonation, not detect it. I don't know if that's possible, but it's worth investigating. Attempting to calculate what the CP might be, based on timing and EGT, seems a little sketchy to me - I think you need to add a few more inputs to that equation (charge temp, RPM, AFR, load) and getting all of the coefficients correct sounds hard. But it couldn't hurt to try.

 

You seem to be assuming that detonation is caused by CP going above some threshold. I don't know if that's true or not, but I suspect not. On the other hand, if you end up with a formula that can accurately predict (model) the detonation threshold, that would be great - whether or not the output of the formula actually corresponds to CP.

 

There are a couple of companies that make CP sensors, integrated with the spark plug of your choice... but they cost around $1000/cylinder and you need to add data acquisition hardware on top of that. It's tempting, but not $1500 tempting.

[ClimberDHexMods]

Good thoughts. My only edit is to assume no detonation threshold (E85 or race gas). I have heard through the rumor mill (same rumor mill that says rear O2 sensor only does long-term fuel changes) that cylinder pressure starts to go up exponentially at a certain point that may or may not be around MBT. A have seen a number of tuners all claim to stop adding timing when torque stops increasing because "it will take a lot of stress of the bearings"... as to say the cylinder pressures start to get crazy despite little gain to torque due to the advanced combustion resisting the rise of the piston stroke to TDC. I have no first-hand experience on this kind of thing. But I would love to hear about someone who has actually logged cylinder pressure against a number of aforementioned variables including ignition timing.

[NSFW]

http://www.tfxengine.com/

[ClimberDHexMods]

bump for hope someone can lend more information to this

[BAC5.2]

Cylinder pressure as it relates to stroke is all you want to know?

 

Above your pay grade doesn't begin to describe the complexity of accurately calculating cylinder pressures in a turbocharged engine. To put it simply, it can't be done. You can arbitrarily estimate some things, and do a little napkin math, but to get an accurate representation of cylinder pressures can't be done, at least by hand. Explosions are quite interesting, and notoriously difficult to actually model. The number of variables that exist in calculating pressures of a contained explosion whose control volume is constantly changing, puts this type of problem into the realm of "holy shit". You have 4 dimensions of flame propagation with a known point source. But in an engine, you don't have a known point source. You have an electrical arc, which varies in 4 dimensions as well. Add in the atomization of fuel (which may or may not be uniform, and is most certainly NOT sufficiently distributed within the cylinder, let alone the evaporation rate as some fuel might remain liquid vs evaporating), the fact that the inside of a cylinder isn't "calm air", and a handful of other variables.

 

The best you can do is simplify everything, and know that your final number will be wrong. You can assume point-source ignition, assume spherical perfect propagation, assume "calm air" inside the cylinder, and assume perfectly distributed and atomized fuel. And, after you do all of that, assume that the engine isn't getting bumped around, and that it doesn't have 3 other cylinders bumping it, or that it has flow disruption from the movement of valves, etc.

 

So, now that your bubble has been popped, lets move on.

 

OEM's typically monitor cylinder pressures using pressure transducers mounted in the head. That seems to work pretty well, and gives a good idea of what's actually happening within the cylinder.

 

An option that might be helpful would be affixing strain gages to the outside of the cylinder liner. By measuring strain in the cylinder, you would be able to accurately calculate pressure in the cylinder because you accurately know geometry, materials, and strain. The problems with this are simple. Strain gages are extremely sensitive to heat, and not entirely durable when exposed to chemical baths (such as coolant). Similarly, you'd have to run wireless strain gages, which will have an inherent lag time and slower sampling rate when compared to wired versions. AND, you'd have to have an enormous number of them, in order to even consider data received as reliable.

 

If your idea is to use cylinder pressures to predict detonation, I don't see why you couldn't... sort of.

 

For cylinder pressure to indicate pre-detonation, you would have to constantly monitor cylinder pressures and compare to a table (likely one with 15 to 25 dimensions, rather than the "simple" 3-dimensional tables you might be used to tuning).

 

Assuming you COULD monitor fast enough to actually make it possible, and that you COULD compute fast enough to catch an errant cylinder pressure, what can you do about it? If pressure is out of line, you can't just arbitrarily open a valve or otherwise quench the combustion process.

 

At the end of the day, if you want to correlate cylinder pressure to a single variable, you'll be disappointed. It can't be done. Too many variables. For example. Pressure vs. ignition timing is totally dependent on almost every other variable. Change intake temperature, change pressure for a given timing.

 

OEM's monitor cylinder pressures only to ensure that, through their range of testing, under no condition do cylinder pressures rise above safe levels (safe in a mechanical sense).

[ClimberDHexMods]

Thank you for your post.

 

OEM's monitor cylinder pressures only to ensure that, through their range of testing, under no condition do cylinder pressures rise above safe levels (safe in a mechanical sense).

 

This is my intention!

 

I have no intention of plotting cylinder pressure in respect to detonation.

 

My concern is that doubling factory load with lots of modifications and then cranking up the timing advance may have a lot to do with what kills a detonation-free engine. My interest is in finding out more about this complete unknown in order to optimize my torque values against cylinder pressure.

 

Intuitively, igniting the chamber say 20 degrees ahead of TDC at max torque is, very generally speaking, going to make plenty of torque. Yet there should be quite a bit of resistance to the stroke reaching and passing TDC, which I don't imagine can be very good for these engines. If these engine could handle it, I wouldn't see a problem with it, but there is definitely a problem.

 

My dream in all this is to change the dialog such that engine safety limits are measured in peak and average cylinder psi, not in wtq and whp. tq and hp have very clearly failed to stand as predictors of engine failure, as far as I can tell.

 

I realize this dream isn't going to catch on, but then for the minority who don't sneeze at it, it could be valuable to have more information on tap. Would love to buy those spark plug sensors, or at least one of them.

[BAC5.2]

What fails in detonation-free engines?

[ClimberDHexMods]

What fails in detonation-free engines?

 

bearings spin, or dare I say more often piston ringlands fracture. Tough to confirm anything because everyone is so guarded about their baby pooping its pants, with logs hard to come by because everyone has a perfect tuner.

[BAC5.2]

I can't see how you could conclusively attribute either to cylinder pressure.

 

I don't believe you could prescribe some "reliability" rating to an engine based on cylinder pressures, and I think this for a few reasons.

 

1) No one is going to modify their heads to monitor cylinder pressures.

2) A single det event throws your hard work out the window. It's generally accepted that exceeding the peak pressure of a vessel renders it unreliable to hold that pressure again. So every det event forces the "limit" of cylinder pressure down.

3) A single cylinder pressure doesn't blanket cover a motor for reliability. You'd first have to assume that a no-detonation engine failure ONLY occurs above a certain cylinder pressure, which I assure you is not the case.

[ClimberDHexMods]

How about approaching it this way:

 

Assuming no detonation (good fuel), let's say I can make 500 btq. Let's say I peg AFR and can make that torque with 30psi and retarded timing, or 20psi and advanced timing. Would it benefit me to know what Cylinder pressure is all through the combustion process under different variable sets? I suspect it would, as there would certainly be a less straining way to produce the same torque.

[BAC5.2]

But cylinder pressure can change when a fly farts near the intake. Or if you pass through the oil cloud of some busted civic, or the fuel cloud of a big truck.

 

Why couldn't you just closely monitor fuel?

 

Efficiency determined by power output per unit fuel would certainly give you the best results, and you could do that without tapping the head for a pressure sensor.

 

I might be wrong in how OEM's use pressure sensors, but I don't think it is a direct component of tuning. I think it's used more as a "safety net" than a primary source of engine data.

[NSFW]

Intuitively, igniting the chamber say 20 degrees ahead of TDC at max torque is, very generally speaking, going to make plenty of torque. Yet there should be quite a bit of resistance to the stroke reaching and passing TDC, which I don't imagine can be very good for these engines. If these engine could handle it, I wouldn't see a problem with it, but there is definitely a problem.

 

Phil from Element Tuning has said in tuning threads at NASIOC that he prefers to run relatively low ignition advance because it results in less "pre-TDC" combustion time (my words, not his). It makes sense intuitively, and he's got a race car to test his theories on, so it appeals to me. He once said something like "don't use any more timing than you need," so I asked, "how do you determine how much timing you need?" and he went silent.

 

My dream in all this is to change the dialog such that engine safety limits are measured in peak and average cylinder psi, not in wtq and whp. tq and hp have very clearly failed to stand as predictors of engine failure, as far as I can tell.

 

I am not sure if this would actually be illuminating.

 

You can kill a Subaru motor with the stock turbo and a crappy tune, and you can get lots of miles out of a Subaru motor with double factory power and a good tune.

 

Cylinder pressure during detonation with a stock motor is more than twice as much as cylinder pressure during normal combustion. In other words, a twice-as-big turbo, if tuned well, will not make as much cylinder pressure as a stock turbo when detonating.

 

So, I think there's basically two levels of cylinder pressure that are interseting: normal combustion, and detonation. I don't think they overlap, but if anyone has evidence to the contrary I'd love to see it. So basically I think that while cylinder pressure measurements would be interesting in their own rights, I think that the really important thing is to know whether there's detonation or not, and you don't need cylinder pressure sensors to know that.

 

I have the same goals as you have, regarding information to use for tuning Subaru motors for high power and longevity. But, I think it would be more practical to use a microphone to listen for knock. I don't think either approach would tell you that you have knock until after the fact, but microphones are cheap. Cylinder pressure would give some additional useful information (and just plain interesting information), but it's prohibitively expensive. Some day I'd like to try it, but I think I'll try a microphone first.

[NSFW]

bearings spin, or dare I say more often piston ringlands fracture. Tough to confirm anything because everyone is so guarded about their baby pooping its pants, with logs hard to come by because everyone has a perfect tuner.

 

For a while, every time I saw a blown-motor thread at NASIOC, I asked how the owner was monitoring knock. In all cases but one the answer was "I wasn't." My guess is that detonation kills most motors, assembly error kills a few more, random part failure kills a few more, and very very few die for any other reasons.

[NSFW]

3) A single cylinder pressure doesn't blanket cover a motor for reliability. You'd first have to assume that a no-detonation engine failure ONLY occurs above a certain cylinder pressure, which I assure you is not the case.

 

In general, or with turbo Subaru motors? Either way, I'm curious about your reasoning and/or evidence.

[BAC5.2]

Motors fail for reasons besides detonation and cylinder pressure. You can't say a motor will only fail if cylinder pressure exceeds some nominal value. There are LOTS of things that can destroy a motor, cylinder pressure alone is likely not very high on the list.

[NSFW]

How about approaching it this way:

 

Assuming no detonation (good fuel), let's say I can make 500 btq. Let's say I peg AFR and can make that torque with 30psi and retarded timing, or 20psi and advanced timing. Would it benefit me to know what Cylinder pressure is all through the combustion process under different variable sets? I suspect it would, as there would certainly be a less straining way to produce the same torque.

 

Phil, as mentioned above, would opt for more boost and less timing. It seems reasonable to me.

 

I want lots of power (400whp or so (Dynojet)) and I want reliability. So, start with upgrades that should be good for about that much power. I advance timing until I find knock, then back it down a couple degrees. I do a ****load of logging to get that not-quite-knock setting at several boost levels, not just WOT pulls at full boost. I don't know of a better way to do tune. With that done, I think my motor should last a long time. We'll see.

 

My biggest worry is that periodically touching the knock threshold will either add up to enough damage to weaken the motor, or it will eventually cause me to get one unusually-large knock event that takes out the motor. Subaru's ECU is pretty good about preventing multiple knock events as long as you make timing changes in small increments - it will pull more timing than I added. So that should keep me from knocking much, and it should prevent the motor from experiencing multiple consecutive knock events and the extra heat buildup that would create. So hopefully the motor can withstand my tuning methods. Like I said, we'll see...

 

If I had access to cylinder pressure measurements, I'd mostly just use it like a more-accurate knock sensor. Like I said, I think there's two types of cylinder pressure: the safe kind (probably good for 450-500whp, I'm guessing), and the destructive kind (kills motors at stock power levels), and I don't think there's any overlap between them.

 

Cylinder pressure measurements would however let me know when adding boost (and advancing timing to the knock threshold) isn't yielding more power, and that would be interesting. For now I've mostly just stayed with the same boost levels that most tuners use with my turbo and pump gas. Dyno pulls (incl. road dyno pulls) could tell me the same thing, though pressure measurements might be more reliable. Or they might not - who knows? I bet they're still fairly noisy.

[NSFW]

Motors fail for reasons besides detonation and cylinder pressure. You can't say a motor will only fail if cylinder pressure exceeds some nominal value. There are LOTS of things that can destroy a motor, cylinder pressure alone is likely not very high on the list.

 

I see what you mean. I was thinking strictly in terms of what causes turbo Subaru motors to die when people hop them up (which seems to be where ClimberD is going), not just motor failures in general. But yeah, there's a lot of ways for motors to die...

 

Turbo Subaru motors mostly fail in ways that do seem to be related to cylinder pressure, though. Cracked ringlands are probably the most common failure mode, and there's plenty of evidence that they're usually a side-effect of detonation. Subaru was delivering cars with bad factory tunes that caused a rash of broken ringlands, which was traced down (to my satisfaction, anyway) to staying at 14.7ish AFRs well into boost, with lots of attendant knock. Subaru issued a recall for reflashing, and the problem faded away pretty quickly. IIRC, that started with the 07 STI, and ended in 08 or so.

 

Spun bearings are probably next on the list, and people I trust attribute that to cylinder pressure as well, barring obvious stuff like plugged oil paths or over-revs. High pressure = crankshaft flex = squeezing through the thin film of oil that the crank normally rides on = metal-on-metal contact = dead bearing.

 

Turbo Subaru motors seem to be a lot more prone to knock than most, and a lot more prone to failure from its effects. And they're also a lot easier for random people to retune. So we see a lot of poor tunes, and a lot of dead motors. And also a lot of significant power increases with stock long-blocks, that last for a long time. So, there's plenty of reason to believe that the motors are strong enough to withstand high power, but not strong enough to withstand detonation. I'm basically betting my motor on the theory that knock is the determining factor.

[ClimberDHexMods]

Maybe my conception of cylinder pressure is all wrong. Yet, I can only figure that lighting off a spark anywhere near MBT (with high octane fuel) is already going to really put cylinder pressure at a much higher point through the manority of the stroke. This is because you're burning more fuel in the chamber instead of outside it, and that extra utilized combustion energy isn't proportional to the increase in torque. For instance, a 2% increase in torque could mean a larger % increase in pressure. This means increasingly pressure is being wasted resisting the piston's rise to tdc, as well as after TDC. More pressure all around. It seems like graphing a timing range at fixed intervals would generally indicate a trend in cylinder pressure too, and knowing these cylinder pressure deltas would give some basis to tuning in this way. Anyone know of a case study of some sort?

[NSFW]

I've got a book somewhere with some interesting graphs... I'll look for it later though, time for bed now.

[ClimberDHexMods]

http://www.tfxengine.com/

 

This link has some fun things

 

Wish I had the money to blow.